8.5 Variation of ice crystal size, shape and asymmetry parameter in tops of tropical deep convective clouds

Wednesday, 9 July 2014: 11:30 AM
Essex Center/South (Westin Copley Place)
Bastiaan van Diedenhoven, Columbia University and NASA/GISS, New York, NY; and A. M. Fridlind, A. S. Ackerman, and B. Cairns

We present an analysis on the variation of ice crystal properties in tops of tropical deep convective clouds. A newly developed remote sensing method is applied that uses POLDER satellite measurements of multi-directional polarized reflectance to infer the aspect ratio and distortion level of ice crystal components in cloud tops, which, in turn, yields the ice crystal asymmetry parameter. Moreover, these retrievals allow for subsequent inference of cloud optical thickness and effective radii from collocated MODIS measurements that are not biased by a preselected, fixed ice optical model. In this presentation, we focus on deep convective clouds observed off the north coast of Australia during a 36-day period that encompasses the TWP-ICE campaign in 2006. Based on precipitation rate, vertical and zonal winds and relative humidity profiles, four periods are identified with specific atmospheric states. We show that retrieved ice crystal shape, asymmetry parameter and effective radii vary systematically with cloud-top temperature, but also with changes of monsoon activity. Although cloud top ice crystal properties are found to vary slightly between periods with strong active monsoon, suppressed monsoon and break conditions, a fourth period with weaker active monsoonal conditions and weak vertical wind shear shows substantially more distorted crystals with effective sizes that increase with decreasing cloud top temperature at over twice the rate compared to the other periods. We will provide some physical interpretations of the results and discuss radiative implications.
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